Expiration of acknowledgements is a feature that is only available when using the Checkmk micro core (cmc, available only in the enterprise edition) instead of Nagios as monitoring core. However, the information on this dependency is rather “hidden” in the onscreen help – while the input fields for expiration after x days/hours/minutes are always visible, even in a non-cmc installation.
People who do not have the onscreen help active, try to use that feature. And those who are not running cmc (most notably the raw edition, of course, but also cee installations using Nagios as core for whatever reasons) will expect to get notified later on when their acknowledgement expires – which it never will.
That’s quite a dangerous pitfall which looks like a grave bug to new users:
To avoid confusion, the input fields should preferrably not be shown when not using cmc. If this is not easily possible, then at least the notice should always be visible (just like the notice for repeated downtimes already is).